What is avogadro's number, and why is it useful?

Explanation:

Substance are consist of very large number of atoms, ions , molecules etc.

They can not be count in a simple way. Scientist used Avogadro number to express the large number of atoms, ions, molecules.

Avogadro number:

It is the number of atoms , ions and molecules in one gram atom of element, one gram molecules of compound and one gram ions of a substance.

The number 6.022 × 10²³ is called Avogadro number.

For example,

18 g of water = 1 mole = 6.022 × 10²³ molecules of water

1.008 g of hydrogen = 1 mole = 6.022 × 10²³ atoms of hydrogen

96.06 g of SO₄²⁻ ion = 1 mole of SO₄²⁻ ion = 6.022 × 10²³ ions of SO₄²⁻

the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula

(that's what i know about formula mass)